Work-around for updating styles to a template that is imported into Campaign Monitor.

cm-styles-multiline-workaround.html

  <multiline label="styles-override">
    <style type="text/css">
      .class {color: red;}
      @media screen and (max-width: 479px) {
        [class="class"] {color: blue;}
      }
    </style>
  </multiline>

<p>or</p>

<style type="text/css">
  <multiline label="styles-override">
    .class {color: red;}
    @media screen and (max-width: 479px) {
    [class="class"] {color: blue;}
    }
  </multiline>
</style>